The Crab nebula's strange behavior

A composite illustration of the AGILE satellite and the Crab Nebula imaged by the
Hubble observatory.
[Image courtesy of ASI, INAF and NASA]

In the year 1054, Chinese astronomers witnessed a supernova, or a brightly exploding star in the sky. Today, the remains of that supernova are still very well-studied by astronomers from all over the world. They call it the Crab nebula—and for years, it has been known for its extremely steady production of radiation. In fact, radiation coming from the Crab nebula has been so stable in the past that many astronomers have used the nebula as a reference point to measure other sources of radiation in the sky.

Recently, however, astronomers have caught the Crab nebula acting very strangely. This week in Science, two separate reports describe unique and unexpected gamma-ray bursts coming from the Crab.

First, Marco Tavani and colleagues used the Italian Space Agency's AGILE satellite to detect a strong gamma-ray flare coming from the Crab nebula in September 2010. These researchers say that this sudden flare-up of gamma-ray activity was just like another remarkable—though still unexplained—flare from the Crab nebula that was detected by AGILE back in October 2007. In light of their findings, Tavani and his colleagues suggest that one or two strong gamma-ray flares may occur in the Crab nebula each year.

In a separate report, a group of researchers led by A. Abdo also describe the September 2010 flare along with another earlier flare that occurred in February 2009. (This February flare happened before the AGILE satellite began looking at the Crab nebula.) These researchers used the international Fermi Gamma-ray Space Telescope to make their observations.

Abdo and the researchers suggest that these strange gamma-ray flares coming from the Crab nebula are caused by some of the highest-energy particles in the universe—and if that is true, then these findings might change what we know about particle acceleration here on Earth as well.